Metal CU, TT earth and wiring for SPD

Thanks for the replies, certainly some food for thought.

I had missed 534.4.7 but I think at least some manufacturers don’t recommend SPD’s downstream of a RCD, will need to check again.

I hadn’t thought about build up of leakage current when there are multiple RCBO’s in a board, I wonder how likely this is to occur in a domestic environment, I could see it as a major concern in a office building full of IT equipment.

Looking at the Wylex RCD device from TLC that Chris pointed out, its in a polycarbonate, not metallic case. My understanding that under regulation 421.1.201 this device would be considered as a similar device and would need to be non-combustible in domestic environments? 

Looking at the data it’s not quite clear if the enclosure is wired with the SPD as a stub, if it’s a stub I think I would need an additional 80A fuse to protect it;  this is something I can ask the manufacturer. But it highlights and area I have been struggling with, we are required to make sure that SPD have OCPD following the manufacturers recommendation but it’s far from easy to find this information.

Regarding the need for an RCD as a main isolator for an all RCBO board the OSG provides an example consumer unit for TT use with this configuration in fig 3.6.3 (i). There is a similar recommendation in NAPIT OSG section 3.5, page 102. Of course there will be different opinions on the suitability of the suggested solutions.

But given both documents state that with a metal consumer unit on TT it’s not recommended to combine a standard isolator on the input with dual RCD because of the risk of abrasion to the connecting cables I assume the same conclusion has to be drawn about interconnections for SPD’s.

I am also wondering if I can put third party SDP’s in consumer units, there are certainly SPD’s available from various companies. But if I can’t mix and match standard RCBO’s and CB’s from different manufacturers in a CU why would it be acceptable to introduce a third party SPD? safe option for now is to use items from same manufacturer.

Overall it looks like my best way forward is a time delayed RCD on the input and then probably a dual RCD arrangement to keep costs down and do more research on what OCPD is required for various SPD’s on the market. Most boards pre populated with a SDD that I have looked at do not include a OCPD to protect the SPD but don't state if anything is required. Hager is an example where so far I have failed to find any data on what if any OCPD is required.

It would be fantastic if someone with suitable knowledge and authority would do some joined up thinking and create a document on consumer unit configurations with SPD’s, taking a special look at TT applications. In my opinion (as someone very new to the industry) it’s a complex subject that needs clarifying and making the wrong decisions could increase rather than decrease risk.

Thanks for the replies, certainly some food for thought.

I had missed 534.4.7 but I think at least some manufacturers don’t recommend SPD’s downstream of a RCD, will need to check again.

I hadn’t thought about build up of leakage current when there are multiple RCBO’s in a board, I wonder how likely this is to occur in a domestic environment, I could see it as a major concern in a office building full of IT equipment.

Looking at the Wylex RCD device from TLC that Chris pointed out, its in a polycarbonate, not metallic case. My understanding that under regulation 421.1.201 this device would be considered as a similar device and would need to be non-combustible in domestic environments? 

Looking at the data it’s not quite clear if the enclosure is wired with the SPD as a stub, if it’s a stub I think I would need an additional 80A fuse to protect it;  this is something I can ask the manufacturer. But it highlights and area I have been struggling with, we are required to make sure that SPD have OCPD following the manufacturers recommendation but it’s far from easy to find this information.

Regarding the need for an RCD as a main isolator for an all RCBO board the OSG provides an example consumer unit for TT use with this configuration in fig 3.6.3 (i). There is a similar recommendation in NAPIT OSG section 3.5, page 102. Of course there will be different opinions on the suitability of the suggested solutions.

But given both documents state that with a metal consumer unit on TT it’s not recommended to combine a standard isolator on the input with dual RCD because of the risk of abrasion to the connecting cables I assume the same conclusion has to be drawn about interconnections for SPD’s.

I am also wondering if I can put third party SDP’s in consumer units, there are certainly SPD’s available from various companies. But if I can’t mix and match standard RCBO’s and CB’s from different manufacturers in a CU why would it be acceptable to introduce a third party SPD? safe option for now is to use items from same manufacturer.

Overall it looks like my best way forward is a time delayed RCD on the input and then probably a dual RCD arrangement to keep costs down and do more research on what OCPD is required for various SPD’s on the market. Most boards pre populated with a SDD that I have looked at do not include a OCPD to protect the SPD but don't state if anything is required. Hager is an example where so far I have failed to find any data on what if any OCPD is required.

It would be fantastic if someone with suitable knowledge and authority would do some joined up thinking and create a document on consumer unit configurations with SPD’s, taking a special look at TT applications. In my opinion (as someone very new to the industry) it’s a complex subject that needs clarifying and making the wrong decisions could increase rather than decrease risk.